The present invention is directed to a battery operated device in which a DC motor is powered by a battery formed by a plurality of cells arranged in series. A switch device is located between the battery and the DC motor whereby, initially, a lower battery voltage is supplied to the DC motor via an intermediate tap of the battery and, subsequently, a full battery voltage is supplied via an end tap of the battery so that the rotational speed of the motor can be changed.
A battery operated device of the above type is disclosed in GB-PS 1,395,508. This known device, illustrated in FIG. 1 in more detail, includes a battery 2 made up of a plurality of cells I connected in series and supplied with power via a bridge circuit 3 and connected, via plugs 4, 5, with the secondary side of a mains or distribution transformer 6. A DC motor 8 is connected with the battery 2 by a switch device 7, whereby first a lower battery voltage is supplied from an intermediate tap 7a of the battery, and then a full battery voltage is supplied by an end tap 7b of the battery to the DC motor 8 by the switch device 7 for changing the rotational speed of the DC motor 8.
In switching the rotational speed, however, there is the danger that a short circuit may occur between the intermediate tap and the end tap as a result of an extended arc, particularly in the case of a DC motor with high output. In such a situation, a power lapse would occur, which is unacceptable, especially in manually operated tools.
A power lapse also results when the respective circuits are interrupted as speeds are changed due to switch adjustment.